1. Field of the Invention
The present invention relates to a semiconductor device, a display device, and a light-emitting device, or a method for manufacturing these devices. In particular, the present invention relates to a semiconductor device, a display device, and a light-emitting device each of which includes a circuit having a thin film transistor in which a light-transmitting semiconductor film is used for a channel formation region, or a method for manufacturing these devices. In particular, the present invention relates to a semiconductor device, a display device, and a light-emitting device each of which includes a circuit having a thin film transistor in which an oxide semiconductor film is used for a channel formation region, or a method for manufacturing these devices.
2. Description of the Related Art
Thin film transistors (TFTs) in which a silicon layer of amorphous silicon or the like is used as a channel layer have been widely used as switching elements in display devices typified by liquid crystal display devices. Although thin film transistors using amorphous silicon have low field-effect mobility, they have an advantage that larger glass substrates can be used.
Moreover, attention has been recently drawn to a technique by which a thin film transistor is manufactured using a metal oxide with semiconductor properties and such a transistor is applied to an electronic device or an optical device. For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor properties. A thin film transistor in which a transparent semiconductor layer formed using such a metal oxide is used as a channel formation region is disclosed (e.g., see Reference 1).
Furthermore, a technique has been considered to increase the aperture ratio in such a manner that a channel layer of a transistor is formed using a light-transmitting oxide semiconductor layer and a gate electrode, a source electrode, and a drain electrode are formed using a transparent conductive film with light-transmitting properties (e.g., see Reference 2).
The increase in aperture ratio increases the light use efficiency, the reduction in power and size of display devices can be achieved. On the other hand, in terms of the increase in size of display devices and application of display devices to portable devices, a further reduction in power consumption as well as the increase in aperture ratio is required.
As a metal auxiliary wiring for a transparent electrode of an electro-optic element, there is known a wiring including a metal auxiliary wiring and a transparent electrode that overlap with each other to be brought into conduction on the upper side or the lower side of the transparent electrode (e.g., see Reference 3).
A structure is known in which an additional capacitor electrode provided on an active matrix substrate is formed using a transparent conductive film of ITO, SnO2, or the like and an auxiliary wiring formed using a metal film is provided in contact with the additional capacitor electrode in order to reduce the electrical resistance of the additional capacitor electrode (e.g., see Reference 4).
In an electric-field transistor including an amorphous oxide semiconductor film, a transparent electrode formed from indium tin oxide (ITO), indium zinc oxide, ZnO, SnO2, or the like; a metal electrode formed from Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like; a metal electrode formed from an alloy containing any of the above elements; or the like can be used for a gate electrode, a source electrode, and a drain electrode. It is known that a stack of two or more layers of such materials can reduce the contact resistance and increase the interface intensity (e.g., see Reference 5).
It is known that a metal such as indium (In), aluminum (Al), gold (Au), or silver (Ag); or an oxide material such as indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn2O4), cadmium tin oxide (Cd2SnO4), or zinc tin oxide (Zn2SnO4) can be used for a source electrode, a drain electrode, and a gate electrode of a transistor including an amorphous oxide semiconductor and an auxiliary capacitor electrode. The materials for the gate electrode, the source electrode, and the drain electrode may be the same or different from each other (e.g., see References 6 and 7).
A display device in which a memory is placed in a pixel is considered in order to reduce power consumption (e.g., see References 8 and 9). Moreover, in the display devices in References 8 and 9, pixel electrodes that reflect light are used.